Planar microcircuits are often used in high-frequency applications. A planar microcircuit is typically fabricated on an alumina substrate, sapphire substrate, or semiconductor wafer using thin-film or thick-film techniques. Planar microcircuits are also fabricated on circuit boards. One or more planar microcircuits are mounted in a package to form a microcircuit module, also commonly referred to as a packaged microcircuit.
Planar microcircuits use planar transmission structures, such as microstrip or coplanar waveguide (“CPW”), and high-frequency signals are typically routed between microcircuit modules using semi-rigid coaxial cables. Planar microcircuits are typically packaged in metal, cavity-type packages with coaxial connectors or feed-throughs that provide transitions from the planar transmission structures to the coaxial cables. A center pin of the coaxial feed-through extends into the interior of the packaged microcircuit over a planar transmission structure, and the center pin is subsequently electrically connected to the planar transmission structure using solder, conductive epoxy, or a ribbon or wire bond.
When the package is machined, a socket for the coaxial feed-through is positioned high enough to account for fabrication tolerances in the machining of the socket, variations in coaxial feed-through, thickness of the planar microcircuit, and thickness of solder or conductive epoxy used to attach the planar circuit to the package. The height of the coaxial feed-through, and hence the center pin, is fixed by the position of the socket and cannot be adjusted. Unfortunately, the height of the center pin of the coaxial feed-through might not be optimum for the electrical performance of the packaged microcircuit.
A package for a planar microcircuit can be quite complex, requiring significant design and fabrication time. Unfortunately, changes to the planar microcircuit might also require a change to the package, triggering another round of package design and fabrication, and often resulting in scrapping previously fabricated packages.
The complexity of the package can significantly increase if several planar microcircuits are incorporated into a single package. Such a package is not usually suitable for testing only one of the microcircuits. Waiting for all the microcircuits to be designed and fabricated before being able to test any of them can significantly slow the development time. Similarly, if all the microcircuits are tested in a single package, it might be difficult to isolate problems in a particular microcircuit.